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In the preceding lab sessions, we’ve been examining hair, which of course is a type of fiber. But hair is by no means the only type of fiber that may have forensic significance. Many natural and artificial fibers are used in clothing, rugs, carpets, draperies, and other home furnishings, packing materials, building materials, rope and cord, and so on. Such fibers may be transferred between the victim and the criminal, or between a person and the environment at a crime scene or other significant location.

There are five main classes of fibers that are of interest to forensic scientists:

Animal fibers

Animal fibers, such as wool or mohair, are processed versions of raw animal hair and retain some or all of the characteristics of the raw animal hair. Silk, which is the dried exudate produced by silkworms, is not technically a hair but is classed as an animal fiber and shares many of the characteristics of fibers produced from animal hair. (Although spider silk is very similar to the silk produced by silkworms, we were unable to find any cases in the literature in which the forensic examination of spider silk played a prominent role.)

Plant fibers

Plant fibers, such as cotton, linen, sisal, and hemp are very widely used, and the forensic examination of these fibers frequently yields important evidence in criminal cases. Plant fibers may undergo little processing, such as the hemp used in rope and the sisal used for baggage, or they may be highly processed, such as the mercerized cotton used in clothing. Regardless of the production method used, plant fibers retain characteristic chemical and physical properties that can be used by forensic examiners to identify and match specimens.

Artificial fibers

Artificial fibers, also called synthetic fibers, include such familiar fibers as nylon and polyester. These fibers are essentially plastics in fiber form. They are widely used in clothing and other fabrics as substitutes or supplements for natural fibers, either because they are less expensive than the natural fiber they replace or because their physical characteristics are superior to those of natural fibers. Artificial fibers may mimic the properties of natural fibers quite closely, and may be difficult to distinguish from similar natural fibers without close examination. Under microscopic examination, artificial fibers are easily discriminated from natural fibers because the artificial fiber, being machine-made, is absolutely consistent and does not show the variations present in all natural fibers.

Reconstituted fibers

Reconstituted fibers, also called semi-synthetic fibers, such as rayon, are manufactured from cellulose and other natural raw materials that have been processed into a raw liquid that is then reconstituted as fibers using the same types of equipment used to produce artificial fibers. Accordingly, reconstituted fibers resemble natural fibers chemically, but resemble artificial fibers microscopically.

Mineral fibers

Mineral fibers, notably asbestos, are sometimes important forensically. These fibers are easily discriminated from all other types of fibers based on their physical properties alone. In essence, they are rock in fiber form. Although fiberglass is a manufactured product, it is often considered a mineral fiber.

The first task a forensic scientist faces when presented with a questioned fiber is to identify the class to which that fiber belongs. The oldest method for determining fiber class, and one that is still sometimes used today, is to burn a small specimen of that questioned fiber to determine what the smoke, if any, smells like. Using this test, you can readily discriminate among most of these classes of fiber, as follows:

Plant fibers burn readily and produce an odor similar to that of burning paper.

Artificial fibers melt as they burn and produce an acrid odor of burning plastic.

Reconstituted fibers burn with the same odor as their natural precursors (typically cellulose) and so cannot be discriminated from natural plant fibers.

Mineral fibers do not burn or produce any odor.

Using the burning test eliminates all but one or two fiber classes from further consideration. If your nose is sensitive, you may even be able to discriminate different fibers within the same class. For example, after some practice, most people can discriminate the odor of burning wool from that of burning silk or that of burning nylon from that of burning polyester. If your nose is very sensitive, you may even be able to identify blends, such as a 60/40 cotton/polyester shirt fabric.

Figure 6-12. Barbara test-burning a fiber specimen

In this lab session, we’ll burn fiber specimens and record our observations.

Required Equipment and Supplies

goggles, gloves, and protective clothing

gas burner or butane lighter

forceps or tweezers

fiber specimens (see Substitutions and Modifications)

The Maker Shed carries a variety of chemistry supplies and equipment. Check out the Science Room section to see what’s currently available.

CAUTIONS

Although none of the activities in this lab session present any significant risks, as a matter of good practice you should always wear splash goggles, gloves, and protective clothing when working in the lab, if only to avoid contaminating specimens. Obviously, you may need to work without goggles when using a microscope or magnifier to examine specimens.

Ideally, each specimen should be of a single type of fiber, although you can also test blends. Obtain specimens large enough for the other tests in the following lab sessions–at least a few square centimeters–and label each specimen or store it in a labeled container. One good source of known fiber specimens is discarded clothing, which is nearly always labeled with the fiber content. You can also obtain specimens from hems and other hidden areas of your current wardrobe. Craft and fabric stores are another good source of known fiber specimens.

Small specimens of Multifiber Fabric #43, suitable for use in the lab sessions in this chapter, can be purchased directly from Testfabrics, Inc.

Procedure

If you have not done so already, put on your goggles, gloves, and protective clothing.

Grasp the first specimen with your forceps. Burning even one fiber may provide a strong enough odor to be characterized by an experienced examiner, but until you get some experience we recommend using a specimen about 0.5 to 1 cm square.

Ignite the burner or lighter, and bring the fiber close to, but not into contact with, the flame. Does the fiber ignite, curl, or melt? Note your observations in your lab notebook and/or in Table 6-10.

Touch the end of the fiber to the flame. Does the fiber ignite immediately, slowly, or not at all? Does it simply melt, or is there no apparent change? If it burns, does it burn quickly or slowly, smoothly or with a sputtering flame? Note your observations in your lab notebook and/or in Table 6-10.

Remove the fiber from the flame. Does it continue burning or extinguish? If it continues burning, is there an open flame or does it smolder with a glowing tip? Note your observations in your lab notebook and/or in Table 6-10.

Repeat steps 2 through 5 for each of your fiber specimens.

If you have time, ask someone to choose a questioned specimen for you from among your fiber specimens. Then do a burning test of that questioned specimen and attempt to identify it. Olfactory memory is notoriously unreliable, so don’t hesitate to repeat burning tests on your known specimens as necessary when you are attempting to identify the questioned specimen.

Fiber Preparation

Quite often, fibers can be tested without any preliminary treatment. In some cases, however, the presence of oil, starch, wax, or some other surface coating may interfere with some tests. Preliminary cleaning of the fibers, called boiling off, is usually a simple matter of boiling the fibers for a few minutes in distilled or deionized water. If that treatment fails, try using a warm dilute (~0.1 M) solution of hydrochloric acid or sodium hydroxide, followed by a thorough rinse.

Table 6-9. Burning characteristics of fibers

Fiber

Melts?

Curis?

Burns?

Extinguishes?

Residuum

acetate

■

■

■

□

hard black bead

acrylic

■

■

■

□

hard black bead

aramid

□

■

■

■

hard black bead

azlon

■

■

■

□

hard black bead

cotton

□

□

■

□

white or gray ash

glass/mineral

■

■ / □

□

■

hard colorless bead

modacrylic

■

■

■

■

hard black bead

nylon

■

■

■

□

hard black bead

nytril

■

■

■

□

hard black bead

olefin

■

■

■

□

hard brown bead

polyester

■

■

■

□

hard black bead

rayon

□

□

■

□

none

saran

■

■

■

■

hard black bead

silk

■

■

■

□

soft black bead

triacetate

■

■

■

□

hard black bead

vinal

■

■

■

□

hard brown bead

vinyon

■

■

■

■

hard black bead

wool

■

■

■

□

black ash

Table 6-10. Test Fiber specimens by Burning – observed data

#

Fiber

Observations

1

2

3

4

5

6

7

8

9

10

11

12

Review Questions

Q1: You burn a fiber and detect an odor similar to household ammonia. What type of fiber do you suspect?

Q2: You burn a fiber and detect an odor similar to burning paper. What type or types of fiber do you suspect?

Q3: You burn a fiber, which does not melt as it approaches the flame. It curls as it burns, and it self-extinguishes when you remove it from the flame. A hard black bead forms at the burned end of the fiber. What specific fiber do you suspect?